This invention relates to noise reduction apparatus and more particularly to signal coring type circuitry.
Rudimentary coring circuits consist of parallel back-to-back diodes serially connected in a signal processing path. Signals having amplitudes less than the forward biased diode potential V.sub.BE are blocked by the diodes and large amplitude signals are passed but with their amplitudes diminished by an amount equal to the diode potential V.sub.BE. If the assumption is made that noise attendant the signal is of small valued amplitude, the effective signal-to-noise ratio (SNR) of the processed signal can be improved by such a circuit. The magnitude of the window which blocks the low level signals can be increased or decreased by adjusting the respective DC bias potential applied to the diodes, see for example U.S. Pat. No. 4,009,334 entitled "Video Noise Reduction Circuit".
In a television context, the SNR improvement provided by coring reduces "speckles" in the picture. Such noise is ordinarily removed in an analog corer by a frequency-separation filter for dividing the signal into high and low-frequency components. The high frequency components are thereafter passed through a network including diodes, and an output signal is obtained only for those high-frequency signals exceeding a diode threshold voltage. The cored high-frequency components and the unmodified low-frequency components are then additively combined to form a complete signal in which small-amplitude high frequency components are removed (see U.S. Pat. No. 3,333,055).
The magnitude of the high-frequency component of the signal is ordinarily reduced by the offset voltage of the diodes. Thus, the cored signal has an effective high-frequency signal amplitude reduction, which is an unwanted distortion.
The reason for separating the frequency band in the aforementioned corer is to allow the diode offset voltage to effectively "ride" on the low frequency signal component. That is, the diode corer circuit alone, without the frequency-separation filter, would remove signal components about a baseline but would not remove small-amplitude high-frequency noise superimposed on a large-amplitude low-frequency component.
U.S. Pat. No. 4,219,745, entitled "Backlash Filter Apparatus" and issued to Hersman, describes a coring type circuit for analog signal processing in which the coring window or thresholds floats with or tracks the lower frequency signal content. In this circuit, the coring diodes are arranged in the negative feedback path of a differential amplifier. The coring window V.sub.w is determined by the amplifier gain and given by the relation EQU V.sub.w =2V.sub.BE /gain
and is typically adjusted to be equal to the peak-to-peak noise voltage.
Returning to the video signal context, coring is implemented to reduce speckles in the reproduced signal. The speckles are the visual manifestation of electrical noise riding on the video signal. However, the intensity of visual speckles as perceived is not directly related to peak noise amplitude. For instance, white going noise components in a generally bright scene and black going noise components in a generally dark scene are much less visible than equal amplitude black going noise components in a bright scene and white going noise components in a dark scene. Considering that coring is a distortion of the video signal which can obscure or eliminate fine signal detail, it is desirable to perform the least possible amount of coring while still substantially eliminating the speckles. In conformance with this constraint, and because of the non-symmetric visual manifestation to noise in the video signal, the corer should have a variable window which tracks the signal and should have an asymmetric transfer function. That is, for a video signal representative of a dark scene, the corer should reduce the amplitude or eliminate white going noise components to a greater extent than black going noise components; and similarly for a video signal representative of a brighter scene, the corer should reduce black going noise components to a greater degree than white going noise components. This implies that the coring window be variable and a function of the instantaneous signal magnitude.